A nucleic acid is a biopolymer present in the nucleus of cells of living organisms, and is made of nucleotides linked together by phosphoester bonds. Each nucleotide is made of a sugar, a base, and phosphoric acid. Nucleic acids are broadly divided, depending on the kind of sugar used, into deoxyribonucleic acid (DNA) having 2-deoxyribose and ribonucleic acid (RNA) having ribose. These nucleic acids are deeply involved in storage of genetic information and transmission of information in the cells of a living body.
Further, it is known that both DNA and RNA contain four kinds of bases and are composed of two nucleotide chains forming a double helix.
In recent years, it has become known that intake of nucleic acids has an effect on, for example, cell metabolism, immune function, brain function, and lipid metabolism and provides a skin-lightening effect. In order to utilize such effects of nucleic acids, nucleic acid-containing foods, beverages, health foods, and raw materials for pharmaceutical products are produced and sold.
For example, a thermally-stabilized prostaglandin composition including bioactive prostaglandin and RNA or DNA is known (see, for example, Patent Document 1). The composition is produced by adding prostaglandin in the form of crystal, powder, or solution to an aqueous solution with a pH of 7 to 8 containing DNA or RNA in such an amount that the weight ratio of DNA or RNA to prostaglandin to be added is usually 10:1 to 1000:1, preferably 50:1 to 400:1.
Further, a hot spring water-based composition including liposomes of hot spring water stabilized in a DNA gel is known (see, for example, Patent Document 2).
However, it has not been known that nucleic acids contribute heat-stability to protein.
Lactoferrin (LF) is an iron-binding protein contained in tears, saliva, peripheral blood, milk, etc of a living body. It has been reported that lactoferrin has various physiological activities such as antibacterial effect against harmful bacteria, enteral iron absorption-promoting effect, immunostimulating effect, cell growth effect, anti-inflammatory effect, and bifidobacteria growth effect.
As described above, lactoferrin having such excellent physiological activities is a protein contained in, for example, mammalian milk, and is therefore very safe for ingestion. For this reason, lactoferrin is used in various products, and various lactoferrin-containing products such as foods, beverages, processed foods, pharmaceutical products, and modified powdered milk for infants are commercially available.
However, it is known that lactoferrin is unstable to heat, and particularly when the pH of lactoferrin is in the neutral region, its physiological activities are almost lost by heating at 62.5° C. for 30 minutes and are completely lost by heating at 70° C. for 15 minutes (see, for example, Non-Patent Document 1). Further, it has been reported that the iron-binding ability of lactoferrin is adversely affected by denaturation caused by heating (see, for example, Non-Patent Document 2).
On the other hand, it has been disclosed that when the pH of a lactoferrin solution is in the range of 1.0 to 6.5, particularly in the range of 2.0 to 6.0, lactoferrin is stable to heat (see, for example, Patent Document 3).
As lactoferrin having improved thermal stability, a lactoferrin composition containing soybean polysaccharide, xanthan gum, or the like as a stabilizer has been reported (see, for example, Patent Document 4).    Patent Document 1: JP-B No. S60-19742    Patent Document 2: Japanese Patent No. 3230239    Patent Document 3: Japanese Patent No. 2688098    Patent Document 4: JP-A No. 2004-352669    Patent Document 5: Japanese Patent No. 3223958    Non-Patent Document 1: Journal of Pediatrics, vol. 90, p. 29, 1977    Non-Patent Document 2: Advances in Experimental Medicine and Biology, vol. 357, p. 219, 1994